Add HTTP2 and Allow Custom Handlers

[michaelg100]

Was curious about integrating gRPC with the service this is just a few changes to make possible and then the example:

https://github.com/michaelg100/caesar-start-kit-gprc/tree/main

for actual testing checked by replacing with local in the start kit.

replace github.com/caesar-rocks/core => /../local/core

One note for the http2:

If a request is an h2c connection, it's hijacked and redirected to s.ServeConn. Otherwise the returned Handler just forwards requests to h. This works because h2c is designed to be parseable as valid HTTP/1, but ignored by any HTTP server that does not handle h2c. Therefore we leverage the HTTP/1 compatible parts of the Go http library to parse and recognize h2c requests. Once a request is recognized as h2c, we hijack the connection and convert it to an HTTP/2 connection which is understandable to s.ServeConn. (s.ServeConn understands HTTP/2 except for the h2c part of it.)